The present invention relates to a tractor having an endless belt with driving pins, which feeds documents having perforations along their margins to a given printing position within a printer.
The tractor is used in a printer which is used; as computer peripheral equipment or the like. The tractor is provided in a pair of rows of perforations of a document, respectively. The respective tractors comprise a pair of pulleys spaced-apart in the feeding direction of the document. The pulleys are rotatably supported at both sides of a pair of frames extending in the feeding direction. An endless belt extends between both pulleys, and a large number of pins project from the outer feeding surface of the endless belt at regular intervals.
In the tractor, one of two pulleys is inserted into a driving shaft which is connected to the drive-source of a pulse motor and a D.C. servomotor or the like. The other pulleys or the frames are inserted into a guide shaft, which is provided in parallel to the driving shaft.
The documents are mounted on the tractor so as to cause the feeding perforations punched in both ends to engage the pins of the tractor. Then, when the driving pulley is rotated and driven by the driving source through the driving shaft, the belt is rotated and driven between the two pulleys by the rotation of the pulley. The pins move in association with the rotation of the belt, and thereby the documents are fed to the printing position.
The documents are also in a state such that the central portion in the width direction is loosened by the dead load when the feeding perforations at both ends of the documents are engaged with the pins of the tractor. And the wider the documents, the greater the amount of slackening, and therefore it is liable to generate an inferior quality of printing or paper jams. Therefore, in order not to cause the documents to slacken, the tractor is positioned and fixed in the driving shaft and the guide shaft so as to draw the documents toward the outside width by means of the pins.
At this time, when the force of the tractor to draw the documents is too strong, the feeding perforations are torn or, conversely the portion of the belt positioned between a pair of pulleys is drawn toward the opposite side through the documents, and therefore the belt curves against the feeding direction. In this state, when documents are fed, the feeding perforations will not engage with the pins, and therefore the documents will be fed in a curve to the printing position, or the documents will be damaged, and thus it will be impossible to feed the documents.